I have 64bit CPU but I can't boot/install 64bit guests?

You might see variants of this error: "Host CPU is incompatible with the virtual machine's requirements at CPUID level 0x80000000001 register 'edx'."

64bit guest OSes require additional support from the hardware when executed in the virtual environment. On Intel, this is called Intel VT. On AMD it is AMD-V.

Both Intel VT and AMD-V are hardware based virtualization extensions in the CPU and chipset hardware of the host. Most modern systems should support them, however systems from 2005 or before may or may not have support present. Virtualization features are usually enabled in the BIOS of the hardware, and can be disabled per-VM basis in some virtualization products.

VMware has made a tool available for checking VT status, downloadable at http://ftpsite.vmware.com/download/vt.iso.